1. Field of the Invention
The present invention relates to a fuel injection control apparatus for direct injection type internal combustion engine, especially direct injection and spark Ignition type gasoline engine, which directly injects relatively low pressure fuel in each combustion chamber of internal combustion engine equipped with a plurality of cylinders and capable of controlling fuel injection volume precisely.
2. Description of the Background Art
In order to reduce noxious gas component exhausted from internal combustion engine carried by a vehicle or to improve fuel consumption, fuel injection apparatus for direct injection that directly injects fuel in combustion chamber is utilized and various kinds of technology to control injection volume and fuel injection timing of the fuel injection apparatus have been suggested. For instance, technology disclosed by Japanese Patent Laid-Open No. 312396/1996 bulletin is one example and the technology disclosed by this bulletin includes two types of injection mode and a switching means to switch between these two injection modes which consists of late injection, discharging fuel into combustion chamber during compression stroke of internal combustion engine, and early injection, discharging fuel into combustion chamber during intake stroke of internal combustion engine. On switching the injection mode by switching means, the values of combustion parameter effecting combustion condition is set according to the mode to be activated and injection mode is switched smoothly through control of internal combustion engine which is based on said values of combustion parameters.
As shown in the above bulletin, timing of fuel injection is set so that injection occurs either in compression stroke or intake stroke but fuel injection in compression stroke is intended to form fuel-air mixture having nearly the theoretical ratio in local area nearby ignition plug in a stratified formation which is overall a lean mixture having air-fuel ratio of 30-50. Stratified charge combustion serves to reduce pumping loss or cooling loss when there is light load on internal combustion engine and to improve fuel consumption by means of lean mixture combustion. However, since injection volume increases when there is heavy load on internal combustion engine and air-fuel ratio nearby ignition plug has excessive local density to cause misfire, timing is switched so that fuel injection occurs during intake stroke of internal combustion engine under heavy load to form homogeneous mixture in combustion chamber to make uniform combustion, wherein intake air is cooled by carburetion of fuel injected in combustion chamber so that intake air amount increases and the timing of knocking point advances, resulting in horsepower improvement of internal combustion engine.
In addition, if timing is set so that fuel injection occurs in compression stroke of internal combustion engine, high fuel pressure is required resulting in fuel injection valve becoming oversized. A technique suggested to avoid these problems is the one disclosed by Japanese Patent Publication No. 51893/1989 bulletin. The technique disclosed by this bulletin sets injection pressure of fuel injection valve (fuel pressure) equal to or less than the maximum cylinder pressure of internal combustion engine during motoring and sets the time of the end of fuel injection to be before the time when fuel pressure has the same value with cylinder pressure of internal combustion engine during motoring. Accordingly, fuel injection under low pressure into combustion chamber in the amount required in accordance with the load of internal combustion engine is made possible and the size of fuel pump or fuel injection valve is miniaturized.
None of the conventional fuel injection control apparatus for direct injection type internal combustion engine as mentioned above has fuel adhesion to inlet port, in contrast with multi point injection (so-called MPI) type engine wherein fuel is directly injected in inlet port of internal combustion engine. This enhances the responsiveness of fuel injection control to achieve accurate control of air-fuel ratio. However, in the type of stratified charge combustion engine, since the pressure in combustion chamber is high and the interval from injection to ignition is short, it is required to raise pressure higher than 5 MPa so as to atomize fuel sufficiently. Therefore fuel pump for high pressure type is needed aside from the feed pump and the fuel pipe arrangement must bear high pressure resulting in that mechanical loss increases because of driving fuel pump under high pressure and the improvement of fuel consumption is limited.
In addition, pressure in combustion chamber varies as shown in FIG. 4. In other words, intake valve of internal combustion engine opens 5-10 degrees before intake air top dead center and since the surge tank provided in inlet passage and combustion chamber communicate with each other while intake valve is opened, the internal pressure of cylinder approximately equals to the pressure of surge tank and if intake valve closes at 50-60 degrees after bottom dead center, combustion chamber undergoes adiabatic compression and internal pressure of cylinder suddenly increases so that firing takes place 5-30 degrees before compression top dead center. In the technique disclosed by a Japanese Patent Publication No. 51893/1989 bulletin as a prior art mentioned above, since fuel pressure is set equal to or less than the maximum cylinder pressure of internal combustion engine during motoring and fuel injection ends before cylinder pressure reaches the fuel pressure, end time of injection is during compression stroke so that the pressure difference between internal pressure of cylinder and fuel pressure becomes small during the latter half of the fuel injection period, thereby decreasing the injection amount per unit time and consequently producing error in total injection volume.
Furthermore, as reduction of pressure differential between internal pressure of cylinder and fuel pressure makes injection speed of fuel injected by fuel injection valve slow down, fuel is not atomized sufficiently and combustion becomes worse. In addition, even if it is assumed that first half of a fuel injection period belongs to intake stroke of internal combustion engine, since pressure of the intake air passage fluctuates depending on the load on engine, pressure differential between fuel pressure and internal pressure of cylinder which has almost the same value as intake air passage varies to make a change in fuel injection amount. For example, pressure variation in surge tank due to change in condition of throttle valve of internal combustion engine from idle to full open reaches 0.7 MPa, that corresponds to 2.3% injection amount error wherein the lower the fuel pressure is, the greater the error becomes. Furthermore, when internal combustion engine has low temperature, combustion turns worse due to slow fuel carburetion or increase of injection amount, resulting in that noxious gas are generated or drivability is degraded.
The object of the present invention is to provide fuel injection control apparatus for direct injection type internal combustion engine which controls timing of end of fuel injection so as to be capable of controlling fuel injection volume precisely without raising fuel pressure.
Fuel injection control apparatus for direct injection type internal combustion engine according to the present invention comprises crank angle sensor detecting rotational angle of internal combustion engine, fuel injection valve directly injecting fuel to combustion chamber, fuel pump supplying fuel to said fuel injection valve with supply pressure of fuel being set equal to or less than the maximum cylinder pressure of internal combustion engine during motoring, and control means for controlling open or close timing of said fuel injection valve to control fuel injection volume and fuel injection duration and determining end time of the intake valve of inner combustion engine from the rotational angle of crank detected and output from sensor and controlling fuel injection timing to close fuel injection valve not later than the close timing of intake valve.
In addition, the apparatus comprises pressure regulator comparing pressure of intake air passage with that of fuel supplied by fuel pump and regulating fuel pressure to control pressure differential between the two to maintain a predetermined value.
Furthermore, the apparatus comprises temperature sensor detecting temperature of cooling water, whereby said control means advances timing of closing of fuel injection valve when said cooling water has low temperature.